Phobos may exist a second surface.
Phobos "Fobos". ESA/DLR/FU Berlin (G. Neukum)
Mars is not as big as the Earth, but it also has satellite . Moreover, Mars has two satellites, more than Earth; but Mars' satellites are very small. The first satellite of Mars is called Phobos, with an English name of Phobos, with a width of only 27 kilometers; the second satellite is called Phobos, with a width of only 15 kilometers. The names of these two satellites are from the ancient Greek myth. Phobos is the god of fear, while Phobos "Demos" is the god of panic.
At first glance, Phobos and Phobos are not like native satellites of Mars, but rather like captured asteroids. This is not only the first impression people see their irregular shapes, but their composition is also close to carbon-rich C-type asteroids. However, strangely, the orbits of these two satellites are very regular, very close to the perfect circle, and their orbital plane coincides with the equatorial plane of Mars. This is not like a captured asteroid. The captured asteroid orbit is usually narrow and elliptical, and its orbital plane usually has an angle to the planet's equatorial plane.
So, people have different opinions. Some people think that Phobos are captured asteroids; some people think that Phobos are formed together with Mars; some people think that Phobos are disintegrated from a larger satellite; some people even think that Phobos are Martian spacecraft disguised as natural celestial bodies.
In order to understand the mystery of the origin of the Phoebos, ESA's "Mars Express" probe recently passed Phoebos at a very close distance. In order to achieve better detection results, engineers and technicians modified its procedures in advance to enable its radar gauge to explore the interior of Phobos closer than the designed distance (250 kilometers).
On September 23, the " Mars Express " passed by only 83 kilometers from the surface of Phobos, and in the process, it used its 40-meter-long antenna to emit low-frequency radio waves to Phobos. Most of the radar waves launched by the "Mars Express" were directly rebounded by Phobos' surface, and this part of the echoes showed a bright line in the echo chart; while others entered Phobos. After these radar waves entering Phobos were reflected back, another darker bright line appeared on the echo chart, indicating that there may be some structure with different components inside Phobos.
Due to limited data, researchers cannot determine what this structure is now, nor do they know whether it is related to the origin of Phobos, but only know that it means that there may be another "surface" inside Phobos.
There may be more probes flying to Mars satellites in the future. Japan plans to launch Phobos detectors in 2024, detect Phobos and Phobos, collect Phobos samples and bring them back to the earth, and place a roaming vehicle (surrounder) on the surface of Phobos. The ESA also intends to keep the Mars Express, which has been in service for 19 years, continue to work. The Mars Express is planned to pass by only 24.9 kilometers from the surface of Phobos between 2023 and 2025. We might know more about the origins of these two strange moons, Mars. The Phobos surface radar echo received by the "Mars Express" probe on September 23, 2022. The echo shows that there may be a second surface inside Phobos. INAF - Istituto Nazionale di Astrofisica
Reference
A close encounter with a mysterious moon
https://www.esa.int/Enabling_Support/Operations/A_close_encounter_with_a_mysterious_moon
